Hot dipped galvanized steel sheet with excellent deep drawing properties and ultra-low temperature adhesive brittleness, and preparation method thereof

ABSTRACT

Provided is a hot dipped galvanized steel sheet with excellent deep drawing properties and ultra-low temperature adhesive brittleness, wherein the average diameter of crystal particles of a hot-dip galvanizing layer is 150-400 μm, and the degree of the preferred orientation with respect to the (0001) face of the hot-dip galvanizing layer is 3,000-20,000 cps (counter per second), and a preparation method thereof.

TECHNICAL FIELD

The present invention relates to a hot-dipped galvanized steel sheet,and more particularly, to a hot-dipped galvanized steel sheet havingimproved deep drawing properties and low-temperature adhesivebrittleness, and a method of manufacturing the hot-dipped galvanizedsteel sheet.

BACKGROUND ART

In general, steel sheets are galvanized by passing a steel sheet throughmolten zinc contained in a bath and solidifying the molten zinc appliedto the steel sheet. When molten zinc applied to a steel sheetsolidifies, coarse dendritic crystal grains, called spangles, are formedon the surface of the molten zinc. Generation of such spangles is acharacteristic of zinc solidification.

In detail, when molten zinc solidifies, crystals start to rapidly growfrom solidification nuclei in the shape of dendrites to form the basicstructure of a zinc plating layer, and pools of molten zinc remainingbetween the dendrites are subject to solidification. Due to thissolidification mechanism, spangles are generated. The size of spanglesmay be determined by the basic structure of the zinc plating layerformed in an initial stage of plating.

Such spangles decrease the adhesive strength of paint on a zinc platinglayer and the corrosion resistance of a steel sheet, and even though azinc plating layer is painted, spangles still make the surface of thezinc plating layer uneven and spoils the appearance of a zinc-platedsteel sheet because spangles can be seen through paint.

Therefore, an inorganic salt solution may be sprayed on a steel sheetbefore molten zinc applied to the steel sheet is solidified, so as tominimize the size of spangles formed on the steel sheet. At this time,the inorganic salt solution is sprayed on the steel sheet through anelectrode disposed on the front side of a nozzle. Since the inorganicsalt solution is sprayed through the electrode, droplets of theinorganic salt solution are charged with static electricity and are thuseasily attached to the steel sheet by electrical attraction to minifythe metallographic structure of a zinc plating layer. A phosphatesolution is widely used as the inorganic salt solution.

A plating layer having spangles of 150 μm or less can be formed on asteel sheet by spraying droplets of a phosphate solution charged withelectricity as described above. In this case, the steel sheet can have aaesthetically-pleasing appearance, improved image clarity afterpainting, and high corrosion resistance, and the plating layer can beprevented from breaking into flaking during a press process.

In addition, when molten zinc solidifies, spangles are formed to havedifferent shapes depending on how hexagonal crystals of zinc are formedon the surface of a steel sheet. In other words, since hexagonalcrystals of zinc grow in different angles in different regions of asteel sheet, spangles have different shapes.

FIG. 1A illustrates a hot-dipped galvanized steel sheet on whichspangles having a size of 150 μm or less are formed, and FIG. 1Billustrates a hot-dipped galvanized steel sheet on which spangles havinga size of 400 μm or greater are formed. Referring to the hot-dippedgalvanized steel sheet of FIG. 1B on which spangles having a size of 400μm or greater are formed, relatively large zinc crystals are randomlyoriented, advantageous in terms of brittleness but disadvantageous interms of the appearance of the steel sheet.

However, referring to the hot-dipped galvanized steel sheet of FIG. 1Aon which spangles having a size of 150 μm or less are formed, thespangles have the same crystallographic orientation in a manner suchthat the basal planes of zinc, (0001) planes, are parallel to thesurface of the steel sheet. The above-mentioned crystallographicorientation of zinc in which the basal planes of zinc crystals areparallel to the surface of the steel sheet is known to be most effectivein preventing corrosion, a black patina, and chemical instability. Untilrecently, there have been many efforts to improve such properties.

For example, Japanese Patent Application Laid-open Publication No.:1999-100653 discloses a technique for adjusting the size of spangles tobe within 60 μm to 1000 μm by spraying mist through a nozzle, andJapanese Patent Application Laid-open Publication No.: 1996-188863discloses a technique for adjusting the size of spangles to be 50 μm orless and the level of surface roughness to be within 0.4 μm to 1.0 μm.In addition, U.S. Pat. No. 4,500,561 discloses a technique fordecreasing the size of spangles to 1000 μm or less by using dropletspassed through an electric field.

Many automobile manufacturers have recently attempted to use structuraladhesives for joining steel sheets, in addition to or instead of usingcommon existing welding methods such as spot welding, for the purpose ofreducing manufacturing costs, improving stability, reducing work time,and making processes eco-friendly.

Unlike mechanical joining methods such as spot welding, structuraladhesives are used after determining whether plated steel sheets can bejoined using the structural adhesives at a low temperature of −40° C.for the case of using automobiles in polar regions. However, if anadhesive is used for a galvanized steel sheet in which the (0001) planesof zinc crystals are parallel to the surface of the steel sheet and thusspangles are not formed, a zinc plating layer may easily be strippedfrom the steel sheet at a low temperature of −40° C. or during a deepdrawing process.

The brittleness of a zinc plating layer increases if spangles of thezinc plating layer are small, and the (0001) planes (basal planes) ofthe zinc plating layer function as slip planes or cleavage planes.Therefore, if a zinc plating layer formed on a steel sheet has smallspangles or the (0001) planes of zinc crystals of the zinc plating layerare parallel to the surface of the steel sheet, the zinc plating layermay easily be stripped from the steel sheet when the steel sheet isimpacted.

Therefore, to deal with the recent methods of joining hot-dippedgalvanized steel sheets using structural adhesives, it is necessary todevelop a hot-dipped galvanized steel sheet having an improvedappearance, deep drawing properties, and adhesive brittleness at a lowtemperature.

DISCLOSURE Technical Problem

Aspects of the present invention provide a hot-dipped galvanized steelsheet having improved deep drawing properties and low-temperatureadhesive brittleness by controlling the structure and grain size of azinc plating layer, and a method of manufacturing the hot-dippedgalvanized steel sheet.

Technical Solution

According to an aspect of the present invention, there is provided ahot-dipped galvanized steel sheet with improved deep drawing propertiesand low-temperature adhesive brittleness, the hot-dipped galvanizedsteel sheet including a zinc plating layer, wherein grains of the zincplating layer have an average particle diameter of 150 μm to 400 μm, andintensity of preferred orientation of (0001) planes of the zinc platinglayer is from 3000 cps (count per second) to 20000 cps.

According to another aspect of the present invention, there is provideda method of manufacturing a hot-dipped galvanized steel sheet, themethod including: applying molten zinc to a steel sheet; adjusting theamount of the molten zinc applied to the steel sheet; spraying anaqueous solution on the steel sheet; cooling the steel sheet; andperforming a skin pass milling process on the steel sheet, wherein thespraying of the aqueous solution includes spraying electrically chargeddemi-water (demineralized water) on the steel sheet.

Advantageous Effects

According to the present invention, owing to the spraying ofelectrically charged demi-water and a high reduction ratio of the skinpass milling process, grains of the zinc plating layer of the hot-dippedgalvanized steel sheet can have a reduced size deviation, the intensityof orientation of the (0001) planes of zinc crystals of the zinc platinglayer can be lowered, and the volume fraction of crystallographic twinsof the zinc plating layer can be increased. Therefore, the hot-dippedgalvanized steel sheet can have improved properties such as deep drawingproperties, bending properties, and adhesive brittleness.

DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic view illustrating the crystallographic structureof a hot-dipped galvanized steel sheet of the related art on whichspangles having a size of 150 μm or less are formed, and FIG. 1B is aschematic view illustrating the crystallographic structure of ahot-dipped galvanized steel sheet of the related art on which spangleshaving a size of 400 μm or greater are formed.

FIG. 2A is an X-ray analysis graph illustrating the crystallographicorientation of (0001) planes of a hot-dipped galvanized steel sheet ofthe related art on which spangles having a size of 150 μm or less areformed, and FIG. 2B is an X-ray analysis graph illustrating thecrystallographic orientation of (0001) planes of a hot-dipped galvanizedsteel sheet of the related art on which spangles having a size of 400 μmor greater are formed.

FIGS. 3A and 3B are images illustrating evaluation results oflow-temperature adhesive brittleness of Comparative Samples 1 andInventive Samples 1.

BEST MODE

Hereinafter, the present invention will be described in detail.

According to embodiments of the invention, crystal grains of a zincplating layer of a hot-dipped galvanized steel sheet have an averageparticle diameter of 150 μm to 400 μm.

If the average particle diameter of the crystal grains is less than 150μm, the hot-dipped galvanized steel sheet may have a beautifulappearance owing to small spangles but the zinc plating layer may haveunsatisfactory low-temperature adhesive brittleness. On the other hand,if the average particle diameter of the crystal grains is greater than400 μm, even though the zinc plating layer may have satisfactorylow-temperature adhesive brittleness, the hot-dipped galvanized steelsheet may have a poor appearance and image clarity, and the zinc platinglayer may easily be separated from the hot-dipped galvanized steel sheetduring a continuous press process due to coarse spangles.

In the embodiments of the invention, it may be preferable that thecrystal grains of the zinc plating layer of the hot-dipped galvanizedsteel sheet have a minimum diameter of 30 μm and the deviation of thediameters of the crystal grains be 40% or less of the average particlediameter of the crystal grains.

If the zinc plating layer includes crystal grains having a diameter of30 μm or less, the crystal grains may be more brittle than surroundingcrystal grains, and thus cracks may start from the crystal grains. Inaddition, when the hot-dipped galvanized steel sheet is bent, the zincplating layer may be separated from the hot-dipped galvanized steelsheet, and thus the formability of the hot-dipped galvanized steel sheetmay be deteriorated.

As described above, in the embodiments of the invention, it may bepreferable that the deviation of the diameters of the crystal grains ofthe zinc plating layer be 40% or less of the average particle diameterof the crystal grains. That is, it may be preferable that the size ofspangles formed on the zinc plating layer be uniform within that range.If the deviation is greater than 40% and thus the size of zinc crystalsis not uniform, when the hot-dipped galvanized steel sheet undergoesplastic deformation, the zinc plating layer may receive non-uniformlyapplied force and may thus be partially separated from the hot-dippedgalvanized steel sheet. Therefore, to prevent problems related toadhesive brittleness, it may be preferable that the deviation of thediameters of crystal grains be 40% or less of the average particlediameter of the crystal grains.

In the embodiments of the invention, it may be preferable that theintensity of preferred orientation of the (0001) planes of the zincplating layer of the hot-dipped galvanized steel sheet be from 3000 cpsto 20000 cps (count per second). When the hot-dipped galvanized steelsheet of the embodiments of the invention was irradiated with X-raysgenerated under the conditions of 20 KV and 10 mA, the intensity ofpreferred orientation of the zinc plating layer of the hot-dippedgalvanized steel sheet was measured to be from 3000 cps to 20000 cps. Indetail, the maximum intensity of the (0001) planes of zinc crystals wasmeasured to be from 3000 cps to 20000 cps (the tilt angle of a samplewas 5°, and intensity values measured at intervals of 5° in a rotationalangle of 0° to 360° were averaged).

Referring to FIG. 2A, the intensity of preferred orientation of ahot-dipped galvanized steel sheet of the related art on which spangleshaving a size of 150 μm or less are formed is greater than 20,000 cps,and referring to FIG. 2B, the intensity of preferred orientation of ahot-dipped galvanized steel sheet of the related art on which spangleshaving a size of 400 μm or greater is formed on is less than 3000 cps.

In the embodiments of the invention, the intensity of preferredorientation of (0001) planes is adjusted to be within the range of 3000cps to 20000 cps. If the intensity of preferred orientation of (0001)planes is less than 3000 cps, it is advantageous in terms of thebrittleness of a zinc plating layer but disadvantageous in terms ofappearance due to coarse spangles. On the other hand, if the intensityof preferred orientation of (0001) planes is greater than 20000 cps, theappearance of a zinc plating layer may be good, owing to small spanglesbut the deep drawing properties and low-temperature brittleness of thezinc plating layer may be deteriorated.

In the embodiments of the invention, it may be preferable that thevolume fraction of crystallographic twins of the zinc plating layer ofthe hot-dipped galvanized steel sheet be 30% or greater.Crystallographic twins may be present in the zinc plating layer when thehot-dipped galvanized steel sheet is processed through a skin passmilling process, and in zinc crystals having a hexagonal close-packed(HCP) structure, crystallographic twins function as an important plasticdeformation mechanism to facilitate a deep drawing process and improvebrittleness characteristics. If the volume fraction of crystallographictwins of the zinc plating layer is less than 30%, plastic deformationmay be less facilitated, and the workability of the hot-dippedgalvanized steel sheet may be deteriorated particularly when the size ofthe zinc crystals of the zinc plating layer is from 150 μm to 400 μm.

A method of manufacturing a hot-dipped galvanized steel sheet will nowbe described in detail according to an embodiment of the invention.

In an embodiment of the invention, a method of manufacturing ahot-dipped galvanized steel sheet includes: applying molten zinc to asteel sheet; adjusting the amount of the molten zinc applied to thesteel sheet; spraying an aqueous solution on the steel sheet; coolingthe steel sheet; and performing a skin pass milling process on the steelsheet.

The spraying of the aqueous solution is performed by sprayingelectrically charged demi-water (demineralized water) on the steelsheet.

In the embodiment of the invention, the applying of the molten zinc isperformed by passing the steel sheet through a zinc plating solution toattach molten zinc to the steel sheet. In the embodiment of theinvention, the applying of the molten zinc is not limited to aparticular method or process. That is, molten zinc may be applied to thesteel sheet using any zinc plating solution and process conditions thatare commonly used for manufacturing hot-dipped galvanized steel sheetsin the art to which the present invention pertains. The zinc platingsolution may include aluminum (Al), antimony (Sb), and/or lead (Pb).However, the embodiment of the invention is not limited thereto. Thesteel sheet may be any kind of steel sheet. That is, any steel sheetused for manufacturing a hot-dipped galvanized steel sheet in therelated art may be used.

In the adjusting of the amount of the molten zinc after the applying ofthe molten zinc to the steel sheet, the steel sheet is air-wiped toremove an excessive amount of the zinc plating solution from the steelsheet. The amount of the molten zinc applied to the steel sheet may beadjusted to be any degree considered appropriate by those of skill inthe art to which the present invention pertains. That is, the amount ofthe molten zinc applied to the steel sheet is not limited to anyparticular degree. For example, the amount of the molten zinc applied tothe steel sheet may be adjusted according to the purpose of the steelsheet.

After the adjusting of the amount of the molten zinc applied to thesteel sheet, the spraying of the aqueous solution is performed byspraying electrically charged demi-water on the steel sheet to solidifythe molten zinc. The electrically charged demi-water is sprayed so as toform a uniform zinc plating layer having uniformly sized spangles. If asolution is electrically charged and sprayed in the form of mist,droplets of the solution collide with a molten zinc plating layer andabsorb heat from the molten zinc plating layer to facilitatesolidification of the molten zinc plating layer. However, if aninorganic solution such as a phosphate solution is sprayed, regions ofthe molten zinc plating layer colliding with nuclear particles such asphosphate nuclear particles may lose heat much more quickly than otherregions. Thus, relatively small spangles may be formed on the regions,and relatively large spangles may be formed on the other regions toincrease the deviation of the sizes of the spangles.

If the deviation in the sizes of spangles is large, the zinc platinglayer of the hot-dipped galvanized steel sheet may not be uniformlystressed during a deep drawing process, and thus cracks may start fromrelatively small spangles. In addition, when the hot-dipped galvanizedsteel sheet is bent, the zinc plating layer may be separated from thehot-dipped galvanized steel sheet. That is, a large deviation of thesizes of spangles may deteriorate the formability of the hot-dippedgalvanized steel sheet.

In the embodiment of the invention, it may be preferable thatelectrically charged demi-water be sprayed through a nozzle at ademi-water injection pressure of 0.3 kgf/cm² to 5.0 kgf/cm², an airinjection pressure of 0.5 kgf/cm² to 7.0 kgf/cm², and a demi-waterpressure/air pressure ratio of 1/10 to 8/10.

If the demi-water is sprayed at a pressure of lower than 0.3 kgf/cm²,spangles may not be minified. If the demi-water is sprayed at a pressureof greater than 5.0 kgf/cm², pitting marks may be formed on the steelsheet while the steel sheet collide with droplets of the demi-water, andthus the appearance of the steel sheet may be spoiled.

It may be preferable that the front side of the nozzle may be charged tohave a voltage of −1 KV to −25 KV. If the front side of the nozzle ischarged to have a voltage of less than −1 KV, electrical attraction maynot be sufficient to minify droplets and spangles. On the other hand, ifthe front side of the nozzle is charged to a voltage of greater than −25KV, spangles smaller than 150 pm may be formed on the zinc platinglayer, and thus deep drawing properties and adhesive brittleness may bedeteriorated.

In the embodiment of the invention, after the spraying of demi-water, askin pass milling process is performed on the steel sheet. During theskin pass milling process, crystallographic twins are formed in the zincplating layer. The skin pass milling process may be performed at anelongation of 5% or less.

In the embodiment of the invention, as described above, it may bepreferable that the skin pass milling process be performed at anelongation of 5% or less. During the skin pass milling process,crystallographic twins are formed, which function as an importantprocessing mechanism in zinc crystals having an HCP structure becausethe HCP structure has few deformation mechanisms. In addition, owing tophysical deformation by the skin pass milling process, the intensity ofpreferred orientation of the (0001) planes of zinc crystals may belowered. In other words, if the skin pass milling process is notperformed, the bonding between the zinc plating layer and the steelsheet may not be firm, and the formability of the steel sheet may not begood. On the other hand, if the skin pass milling process is performedat an elongation of greater than 5%, the properties of the steel sheetmay be deteriorated even though the formability and adhesiveness of thezinc plating layer are improved.

MODE FOR INVENTION

An example of the present invention will now be described in detail.However, the present invention is not limited thereto.

EXAMPLE

Hot-dipped galvanized steel sheets were treated with a phosphatesolution or demi-water under the conditions shown in Table 1 to adjustthe size of spangles. Thereafter, the steel sheets were treated througha skin pass milling process at an elongation of 1.0% and a roll pressureof 200 tons to 240 tons, and the adhesive brittleness, appearance, andimage clarity of the hot-dipped galvanized steel sheets were measured asshown in Table 1.

The hot-dipped galvanized steel sheets were prepared by performing ahot-dip galvanization process on soft IF steel sheets having a thicknessof 0.67 mm to form zinc plating layers on the steel sheets at a platingdensity of 70 g/m².

Sizes and size deviations of spangles formed on the zinc plating layerswere measured and analyzed using an optical microscope and an imageanalyzer before the hot-dipped galvanized steel sheets were treatedthrough the skin pass milling process. The measured and analyzed resultsare shown in the “spangle size” and “spangle size deviation” columns ofTable 1 below. Adhesive brittleness was measured by bonding twohot-dipped galvanized steel sheets with an adhesive for automotivestructural parts (Sealer Terokal 5089 by Henkel Korea, Ltd.), keepingthe bonded hot-dipped galvanized steel sheets at −40° C., impacting thehot-dipped galvanized steel sheets with a wedge, and observingseparation of zinc plating layers of the hot-dipped galvanized steelsheets. In Table 1, O denotes the case where a zinc plating layer wasnot stripped off, Δ denotes the case where 20% or less of a zinc platinglayer was stripped off, and X denotes the case where 50% or more of azinc plating layer was stripped off. Appearance and image clarity weremeasured with the naked eye, and results thereof are denoted as good(O), fair (Δ), and poor (X) in Table 1.

TABLE 1 Spangle Spangle size Preferred Spraying size deviationorientation Adhesive Image No. Solution (μm) (μm) (cps) brittlenessAppearance quality *CS1 Demi-water 150  99 28670 x ∘ ∘ **IS1 Demi-water250  81 10190 ∘ ∘ ∘ IS2 Demi-water 350 126 4800 ∘ ∘ ∘ IS3 Demi-water 400— 3253 ∘ ∘ □ CS2 Phosphate 150 — 44214 x ∘ ∘ CS3 Phosphate 250 106 11850x ∘ ∘ CS4 Phosphate 350 141 4215 □ ∘ □ CS5 Phosphate 700 — 1540 ∘ □ xCS6 — 1000 — 954 ∘ x x *CS: Comparative Sample **IS: Inventive Sample

Referring to Table 1, Inventive Samples treated with demi-water havespangles within a preferable size range, intensity of preferredorientation within the range of 3000 cps to 20000 cps, size deviationswithin a preferable range, good adhesive brittleness, and goodappearance.

Although Comparative Sample 1 treated with demi-water has spangleswithin a preferable size range, Comparative Sample 1 has an unacceptablylarge spangle size deviation, an unacceptably high degree of intensityof preferred orientation, and poor adhesive brittleness. ComparativeSamples 2 to 5 treated with a phosphate solution have unsatisfactoryadhesive brittleness or appearance. Comparative Sample 6, a generalhot-dipped galvanized steel sheet has poor appearance.

FIGS. 3A and 3B are images for evaluating adhesive brittleness ofComparative Samples 1 and Inventive Samples 1. Adhesive brittleness wasevaluated based on whether a blue adhesive remained. Referring toComparative Samples 1 shown in FIG. 3A, as indicated by circles, anadhesive does not remain after zinc plating layers are fractured.Referring to Inventive Samples 1 shown in FIG. 3B, an adhesive remainsowing to improved adhesive brittleness.

1. A hot-dipped galvanized steel sheet with improved deep drawingproperties and low-temperature adhesive brittleness, the hot-dippedgalvanized steel sheet comprising a zinc plating layer, wherein grainsof the zinc plating layer have an average particle diameter of 150 μm to400 μm, and intensity of preferred orientation of (0001) planes of thezinc plating layer is from 3000 cps (count per second) to 20000 cps. 2.The hot-dipped galvanized steel sheet of claim 1, wherein the grains ofthe zinc plating layer have a diameter of 30 μm or greater and adiameter deviation equal to or less than 40% of the average particlediameter thereof.
 3. The hot-dipped galvanized steel sheet of claim 1,wherein the zinc plating layer comprises 30% or more, by volumefraction, of crystallographic twins.
 4. A method of manufacturing ahot-dipped galvanized steel sheet, the method comprising: applyingmolten zinc to a steel sheet; adjusting the amount of the molten zincapplied to the steel sheet; spraying an aqueous solution on the steelsheet; cooling the steel sheet; and performing a skin pass millingprocess on the steel sheet, wherein the spraying of the aqueous solutioncomprises spraying electrically charged demi-water (demineralized water)on the steel sheet.
 5. The method of claim 4, wherein the spraying ofthe electrically charged demi-water is performed using a nozzle at ademi-water injection pressure of 0.3 kgf/cm² to 5.0 kgf/cm² and an airinjection pressure of 0.5 kgf/cm² to 7.0 kgf/cm².
 6. The method of claim5, wherein the spraying of the electrically charged demi-water isperformed at a demi-water pressure/air pressure ratio of 1/10 to 8/10.7. The method of claim 4, wherein the skin pass milling process isperformed at an elongation of 5% or less.